1. Field of the Invention
This invention relates to vapor phase growth of a semiconductor. More particularly, it relates to a method for the vapor phase epitaxial growth of a semiconductor multilayer film on a semiconductor substrate and an apparatus for carrying out the vapor phase epitaxial growth method.
2. Description of the Prior Art
The vapor phase growth method is a method in which a thin single crystalline film layer is deposited on a single crystalline substrate by means of a vapor phase reaction, and this method is widely used in the production of semiconductor elements. It is possible to grow continuously single crystalline layers as a multilayer film on a substrate by means of the vapor phase growth method. The vapor phase growth method in the case of a compound semiconductor is used in the production of semiconductor elements, such as field effect transistors (FETs), Gunn diodes, impact avalanche transit time (IMPATT) diodes, semiconductor lasers, luminescent diodes, and Hall devices. For example, the vapor phase growth of gallium arsenide (GaAs) is well-known (cf. J. M. Durand, "Influence of the Growth Parameters in GaAs Vapour Phase Epitaxy," Philips Journal of Research, Vol. 34, Nos. 5/6, 1979, pp. 177-210).
FIG. 1 is a schematic illustration of a conventional horizontal-type apparatus for carrying out the vapor phase growth of GaAs. The apparatus comprises a reaction tube 1, a heating furnace 2 surrounding the reaction tube 1, an inlet pipe 3 for feeding a carrier gas (e.g., hydrogen gas), a container 4 for accommodating the gallium (Ga) source 5 and being provided with an inlet pipe 6 for feeding arsenic trichloride (AsCl.sub.3) and a carrier gas (e.g., hydrogen gas), a supporting means 7 for a single crystalline substrate 8 of GaAs, and a pipe 9 for feeding a doping agent (e.g., sulfur). A desired temperature profile within the reaction tube 1 is generated by the heating furnace 2. The carrier gas (hydrogen gas) flows from the inlet pipe 3 and another carrier gas (hydrogen gas) entraining arsenic trichloride vapor flows into the container 4 from the pipe 6. The Ga source 5, the hydrogen (H.sub.2), and the arsenic trichloride (AsCl.sub.3) react so that a GaAs epitaxial layer is deposited on the GaAs substrate 8. When the doping agent (sulfur) entrained by the carrier gas (hydrogen gas) is introduced into the reaction tube 1 from the pipe 9, a GaAs epitaxial layer doped with sulfur can be obtained. Therefore, it is possible to form a GaAs multilayer film comprising a GaAs epitaxial layer grown without a doping treatment and a GaAs epitaxial layer grown with a doping treatment on the GaAs substrate.
In a case where the formation of a GaAs multilayer film is repeated by using the conventional apparatus illustrated in FIG. 1, if a GaAs epitaxial doped layer having a doping impurity concentration of 1.times.10.sup.18 cm .sup.-3 or more is grown, a part of the doping impurities are contained in another GaAs layer accumulated gradually on the inside surface of the reaction tube 1 and diffuse into (i.e., contaminate) the Ga source 5. As a result, when a GaAs epitaxial layer without a doping treatment is grown, the contained and contaminating doping impurities are volatile and are incorporated into the layer to be grown so that the resistivity of the obtained GaAs layer is lowered due to the undesired amount of doping impurities. Namely, it is difficult to grow an undoped high-resistance GaAs layer having a good reproducibility. In order to eliminate this disadvantage, the GaAs layer gradually accumulated on the inside surface of the reaction tube 1 and containing the doping impurities should be removed from the surface of the reaction tube and the contaminated Ga source 5 should be replaced with a new Ga source. In this case, problems exist in that the period of time necessary to carry out the vapor phase etching treatment in the reaction tube is relatively long, e.g., from 1 to 2 hours, and in that the Ga source must frequently be replaced.
A GaAs epitaxial layer is formed by means of vapor phase growth under either diffusion controlled conditions or kinetically controlled conditions.
In the case where vapor phase growth is carried out under diffusion controlled conditions, variation in the thickness of the GaAs epitaxial layer formed on a GaAs single crystalline substrate is considerably influenced by the growth conditions, such as the growth temperature, the flow rate of the fed gas, the molar fraction of AsCl.sub.3, and the temperature gradient (temperature profile) within the reaction tube. Therefore, the growth conditions must be controlled very precisely. Accordingly, the reproducibility of the epitaxial layer is poor, and it is difficult to form an epitaxial layer having a high uniform thickness on a single crystalline substrate with a large area. However, a high-resistance undoped epitaxial layer having a better reproducibility can be obtained, as compared to the case in which vapor phase growth is carried out under kinetically controlled conditions.
In the case where the epitaxial layer is grown under kinetically controlled conditions, the above-mentioned growth conditions need not be controlled as precisely as when the epitaxial layer is grown under diffusion controlled conditions and it is possible to obtain an epitaxial layer having a high uniform thickness and a high uniform doping impurity concentration on a large substrate. However, since the molar fraction of AsCl.sub.3 is relatively low, it is difficult to obtain a high-resistance undoped layer having a good reproducibility.
If a multilayer film is grown under diffusion controlled conditions and kinetically controlled conditions alternately created in the conventional vapor phase growing apparatus illustrated in FIG. 1, the time necessary for alternating between the diffusion controlled conditions and the kinetically controlled conditions is not short, and, therefore, defects and extraneous grown layers are generated at the interface between the continuously grown epitaxial layers. Accordingly, in the conventional apparatus, the multilayer film is formed under either diffusion controlled conditions or kinetically controlled conditions.